1. Field of the Invention
This invention relates to method and apparatus for interlocking access to a source of power of an amplifier with the position of a power switch requiring the switch to be retained in a power off position to permit access to the power supply or line voltage test points and, more particularly, to a cover plate slidably retained in an amplifier housing in overlying relation with the power switch and movable between a power on position and a power off position to prevent exposure to the hazards of electrical shock.
2. Description of the Prior Art
In the transmission of cable television signals and data transmission signals through coaxial cables a broadband radio frequency signal transmitted between 5-400 MHz is transmitted with a power signal at 30-60 VAC at 60 Hz. The AC power signal supplies power to the amplifiers positioned at selected points in the cable transmission line for amplifying and conditioning the broadband radio frequency signal.
The amplifiers are sealed units formed by a die-cast aluminum alloy housing having a hinged base cover. A radio frequency module and a power supply module are retained in the housing which protects the components from the affects of weather and hermetically seals the electrical components to prohibit entrance of contaminants into the housing. The housing is connected to a number of input and output transmission cables. The amplifier housing is adapted for mounting on a wall or pedestal by the use of external brackets connected to the housing. Strand clamps are connected to the housing to mount the housing to a power line or similar strand.
With the conventional transmission of cable television signals, the power signal is transmitted at a current in the range between about 10 to 12 amps. Current at this level presents a relatively low shock hazard to operating personnel in the event that the plug-in fuses would be removed from the radio frequency module when the amplifier is being supplied with power. Also, in the event that the radio frequency module is disconnected from the power supply module when the modules are under load there is little or no risk to a shock hazard when the power supply signal does not exceed 12 amps.
With the development of telecommunication systems utilizing CATV transmission lines for connecting subscribers to a number of interactive units, the powering scheme used in conventional CATV systems is not acceptable. The interactive units connected to the cable transmission line require power supplied between 45 to 140 volts AC having a cycle rate of 1 Hz. The amperage for the power signal is at least 15 amps which presents a substantially greater shock hazard to operating personnel than experienced with conventional CATV systems where the power signal does not exceed 12 amps.
In a telecommunications amplifier supplied with a power signal of 15 amps precautions must be taken to prevent removal of the radio frequency module from the power supply module under load. At 15 amps the connectors interfacing the two modules can be damaged when the modules are disconnected under load. The power signal is supplied to the radio frequency module through fuses. The fuses are held in place by clips, and if the fuses are removed under load the operator is exposed to hazardous voltage levels. In addition, precaution must be taken to prevent the removal of the radio frequency module from the amplifier housing when the power is supplied to the module.
Safety interlock systems for preventing access to electrical equipment that presents hazardous exposure to high voltage are known in the art. Protective equipment is positioned during dangerous operating conditions to prevent exposure to high voltages. When the dangerous condition is removed, such as removing the supply of power to electrical apparatus, the protective equipment can be opened or removed.
An example of a safety interlock system for electrical apparatus is disclosed in U.S. Pat. No. 4,659,884. The interlock system includes a sliding door interlocked by a bolt or a slide which is actuated by a magnet and is kept under current in a locked position. A locking lever and an actuating part are brought into engagement with each other by a sliding protective hood causing positioning of the toggle lever within a housing in a closed interlocked condition. In the closed condition current is supplied to the machine. In a second or open position of the lever supply of current to the machine is interrupted.
U.S. Pat. No. 4,652,769 discloses a safety interlock system for a multielectrode device, such as an ion source. The interlock system disconnects high voltage supply without removing low power logic signals to allow the system to continue to operate in various modes so that high voltage can be reinstalled safely upon the reinsertion of a module or upon restoration of an interrupt condition.
In U.S. Pat. No. 4,073,000 a metal-enclosed switchgear includes a door that opens into a switch housing. As long as the door is closed electrical current may be supplied to the components within the enclosure interconnected to the contacts of a switch. When the doors open the switch contacts are opened thereby opening the circuit to permit operating personnel to perform service on the components without risk of electrical shock. As long as the door is closed operating personnel cannot gain access to the components.
U.S. Pat. No. 3,534,186 discloses a control cabinet for an electrical power supply connected to a circuit breaker. The circuit breaker is interlocked with the access doors of the cabinet. The circuit breaker is actuated when anyone of the access doors is open.
A further example of an interlock system for electrically and mechanically operated equipment is disclosed in U.S. Pat. No. 4,931,907 in which a latch in a module engages a keeper on a housing and a switch pin is mounted in the latch handle to engage a lever which in turn engages the keeper mounted on the housing. As long as the lever engages the keeper the module is electrically connected to the housing. Movement of the module out of the housing releases the lever arm from the keeper to release an enable switch which electrically isolates the module from the housing.
U.S. Pat. No. 4,885,436 discloses a switch interlock for an electronic module which permits connection and disconnection of electrical connectors only when the main power switch is off. A switch guard is positioned over a toggle-type power switch and slides over the power switch when the power switch is off. A cut-away portion of the switch guard is blocked from upward movement by the button of the power switch when the switch is on.
U.S. Pat. No. 3,846,703 discloses a noise control system for transmission line amplifiers in a CATV system. Control units are manually actuated by switches. Opening a switch disconnects a return amplifier from its feeder cable and closing a switch connects it to an associated feeder cable.
While safety interlock systems for electrical apparatus are known to prevent operating personnel from being exposed to hazardous voltages, the known systems do not provide a solution to protect in a failsafe manner operating personnel from exposure to risk of electrical shock in the installation and maintenance of telecommunication amplifiers. Because transmission amplifiers are now being used to power a number of broadband network devices higher voltages are required to be supplied to the amplifiers. The amplifiers are readily assembled and disassembled in the telecommunication transmission lines. If precautions are not taken to prevent removal of the respective modules under load or preclude access to fuses under load, serious injury may be inflicted on operating personnel and damage incurred to the electrical components.
Therefore there is need in telecommunications amplifiers for a safety interlock system that prevents in a substantially failsafe method operating personnel from coming in contact with the electrical components under load and when the power is interrupted, power cannot be restored until hazardous voltage conditions are eliminated.